Phosphoric acid is generally produced in accordance with two processes. In one process, wet phosphoric acid is produced from phosphate ore by contact with a strong mineral acid, such as sulfuric acid. Calcium in the phosphate ore is generally separated in the process as gypsum whereas the other constituents in the phosphate ore remain in the phosphoric acid as dissolved solids or soluble salts to the extent they cannot form slightly soluble compounds in the strongly acid medium with either sulfuric or phosphoric acid. In many instances, these impurities interfere with the further use of the phosphoric acid, and often the employment of technical grade acid for many purposes is rendered impossible by these impurities. Efforts have not been lacking, therefore, to free the phosphoric acid or its alkaline salts of such impurities. In one long-used purification method the phosphoric acid is neutralized with alkalis. Accordingly, the impurities dissolved in the raw acid are precipitated as slightly soluble salts or metal hydroxides. This method is therefore suitable as a purifying method only if the phosphoric acid is to be converted into its alkali salts.
Recently, a plurality of extraction methods have been advanced wherein the depletion of impurities from wet method phosphoric acid is obtained by the addition of different organic solvents. Accordingly, phosphoric acid is preferably dissolved in an organic solvent phase while the impurities remain in the water phase. Such extraction methods are divided into two major categories depending on the differences in the physical behavior of the organic solvent with respect to the phosphoric acid. One category uses organic solvents or solvent mixtures which are not miscible with water or only miscible to a limited extent. The other category uses organic solvents or solvent mixtures which are miscible with phosphoric acid and water in all proportions.
Representative of extraction methods of the first category are described in German Pat. Nos. 1,277,221 and 1,162,817; German published application No. 1,294,944; U.S. Pat. No. 3,318,661 and British Pat. No. 1,093,927. Such methods require a certain P.sub.2 O.sub.5 concentration in the raw acid in order to assure extractability of the acid by the organic solvent with phosphoric acid being recovered in a dilute water phase from the organic phase by return extraction.
The second category for purifying wet phosphoric acid utilizes miscible organic solvents, such as methanol, ethanol, propanol, acetone and methyl ethyl ketone. These solvents are homogeneous with phosphoric acid and water in all proportions. By addition of an alkali compound to the phosporic acid, impurities are precipitated when the acid is mixed with organic solvent. Purified phosphoric acid is separated from the solvent by distillation.
In German Pat. No. 648,295, there is described a method for purifying raw phosphoric acid by treating the phosphoric acid with an organic solvent in the presence of an alkaline compounds whereby the impurities, such as calcium, iron, aluminum, chromium, vanadium and fluorine are removed as a precipitate. Mentioned as solvents are methyl and ethyl alcohol and acetone. Alkaline compounds used in the process include free alkali and ammonium base, the salts of sulfuric acid, oxalic acid, carbonic acid, nitric acid and the like. Because of the large amounts of alkaline compounds required for purification, a partial desaturation of the phosphoric acid is effected. Additionally, the impurities settle-out as viscous, green clumps of oily appearance which only gradually become discrete.
German Pat. Nos. 2,032,263; 2,050,008; 2,050,406; 2,053,883 and 2,229,609 also describe methods for purifying wet phosphoric acid utilizing solvents which are miscible with water and phosphoric acid. According to these methods, the solvent is separated from the phosphoric acid by distillation. In German published application No. 1,952,104, there is disclosed a method for purifying wet method phosphoric acid by adding alkali and a water miscible solvent. The clean purified phosphoric acid is separated from the phosphoric acid/solvent/water mixture in the form of a monobasic neutralized salt. The supernatant light phase should consist substantially of water free alcohol, thereby eliminating distillation as the method for separating the solvent. When a less concentrated raw acid is purified, the solvent phase contains a high concentration of water after separation of the phosphoric acid as alkali phosphate. When using concentrated raw acids, a self-solidifying alkali-phosphate melt is obtained, i.e., a solid crystal layer results which is difficult to separate and which can only be eliminated by effecting separation at high temperatures, e.g., 100.degree. C.
In German published publication No. 2,029,564, there is disclosed a method for recovering phosphoric acid in the form of mono-, di-, or tri-alkali orthophosphate from a phosphoric acid/solvent/water mixture. Solvents which are used are able to absorb at least nine percent by weight of water thereby limiting the number of solvents which may be used alone or in admixture with other solvents.
In accordance with British Pat. No. 1,345,710, a method is disclosed for making phosphoric acid and alkali metal phosphates by mixing a wet method phosphoric acid with water miscible solvents, such as acetone, isopropanol and condensated alkali or ammonium phosphate solvents, such as tripolyphosphate, pyrophosphate or long chained polyphosphate solvents. A phosphoric acid/solvent phase and a water polyphosphate impurity phase are formed with the latter phase containing the impurities. The two phases are brought into countercurrent contact by means of a mixer-settler apparatus and are again separated. Free phosphoric acid is separated in a distillation column from the acid/solvent phase if phosphoric acid is to be recovered and alcohol to be recycled to the process.
A different method for obtaining an alkali phosphate salt from a phosphoric acid/solvent phase by eliminating distillation is by treating the organic phosphoric acid with an alkali so that the acid separates from the mixture in the form of an alkali phosphate with a Na/P ratio of at least 1:1.
British Pat. No. 1,345,710 teaches that when neutralizing phosphoric acid with alkali that serious problems are present up to a 1 base or to the 1.67 base alkali phosphate stage due to crystalization tendency which exists in an alkali phosphate base. Continuous extraction methods, therefore, are generally effective at elevated temperatures, e.g. of about 90.degree. C. whereat the temperature must be constantly monitored and controlled. Such a temperature is clearly above the boiling point of the organic solvent of the main component thereby requiring elaborate devices to condensate the lower boiling solvent.